Plates of hardened material, such as steel and steel alloys, are often used to minimise the effect of wear on structural elements of a piece of equipment. Often, these hardened plates are called wear plates because the material of the plate is selected for its resistance to wear. Wear plates are also known to be made of other materials, such as rubber and ceramics. Examples of equipment which use wear plates include hoppers, bins and chutes used in rock handling equipment and in an ore processing plant. This sort of equipment can be exposed to wear in the form of sliding and/or gouging abrasion. The wear plates act as a sacrificial element so that the plates are worn rather than the structural element of the equipment. The plates can be readily exchanged once worn, thus extending the life of the working equipment.
The wear plates are typically square in shape such that they can be tiled across the surface of the structural element to be protected from wear. Each wear plate is provided with four bolt holes, one near each corner.
Recently, harder materials have been used to form a wear plate. Such a harder wear plate is a sheet of very hard material (such as a steel alloy having a Brinell hardness of approximately 500 HB) which is attached to the surface of the equipment which would otherwise be subject to wear. It is possible to perform cutting, welding, drilling and machining operations on such steels, however the difficulty of such operations increases with the hardness of the material. For example, when drilling steel of this hardness a tungsten carbide drill bit is usually required, and the feed and speed rates of the drill bit need to be carefully selected, thus requiring complex expensive drilling machinery.
FIG. 1 shows a cross sectional view of a wear plate 10 with a hole 12 for fixing the wear plate 10 to a structural element (not shown) according to a known system. The surface 14 on the wear plate 10 is exposed to wear. Therefore, the opposing surface 16 will abut the structural element. In this system, a standard counter sunk bolt 18—as shown in FIGS. 2 and 3—is provided. The counter sunk bolt 18 has an externally threaded shaft 20 which extends from a frusto-conical head 22. The head 22 is provided with a hexagonal hole 24 for receiving an Allen key to hold or turn the bolt 18 during fastening etc.
Obviously, the structural element would be provided with a hole for receiving the shaft 20 of the bolt 18. The hole 12 in the structural element could be either an internally threaded hole to threadingly engage the thread on the shaft 20, or could merely be a throughway such that a nut (not shown) can be threaded onto the shaft 20.
The hole 12 in the wear plate 10 is shaped to allow the bolt 18 to be recessed with respect to the wear surface 14. Accordingly, the hole 12 comprises a first cylindrical portion 26, a frusto-conical portion 28 and a second cylindrical portion 30. The first and second cylindrical portions 26, 30 are dimensioned to receive the head 22 and the shaft 20 of the bolt 18, respectively. It should be noted that the frusto-conical portion 28 is dimensioned to compliment the frusto-conical shape of the head 22. Furthermore, as shown in FIG. 2, the half-opening angle θ of the frusto-conical head 22 is 45°.
The bolt 18 is recessed to minimise the amount of wear which the bolt head 22 will experience. This is at a price to the wear plate 10 because the recessing of the head 22 leaves a void in the cylindrical portion 30. This in turn allows material to catch on the wall of the cylindrical portion 30 which increases wear around the hole 12. It is noted that prior to this invention the bolt 18 was a standard “off-the-shelf” bolt, typically having a hardness which is much less than the hardness of more recent harder ware plates.
Due to the properties of the material used in the wear plates 10, forming the hole 12 is an expensive and time consuming process. Generally, the hole 12 can be created in at least two drilling operations using a small drill bit to create the second cylindrical portion 30, and a larger drill bit with a conical tip to create both the first cylindrical portion 26 and the frusto-conical portion 28.
The procedure for lining a piece of equipment with wear plates is, for each wear plate in turn, as follows:    1. the wear plate is located on the surface of the equipment;    2. the four bolts are inserted through their respective holes in the wear plate and through corresponding holes in the equipment; and    3. threading nuts onto each of the bolts.
Given that there can be in excess of 100 wear plates lining, for example, the walls of an ore chute, this is clearly a very time consuming task. Plant equipment downtime ultimately costs the plant due to lost productivity. For this reason it is highly desirable that the time taken to line a piece of equipment with wear plates be reduced.